RESPIRATORY ORGANS. 235 



main pancreatic duct (Wirsung's duct), the dorsal, the accessory 

 or Santorini's duct. The ducts may remain distinct; they may unite 

 before entering the intestine or one of them may unite with the bile 

 duct. 



For a long time it was supposed that a pancreas was lacking in 

 certain vertebrates (some teleosts, dipnoi, cyclostomes) , but recent 

 studies have shown its presence in many of these. In the case of some 



ccp 



FIG. 242. Diagram of developing pancreas of cat, after Thyng. c, ductus coledo- 

 chus; d, duodenum; dp, dorsal pancreas; ddp, its duct; *, small intestine; s, stomach; vp, 

 ventral pancreas. 



teleosts it occurs as a slender tube in the mesentery; in the dipnoi it is 

 outside of the muscles in the intestinal wall, while in the cyclostomes 

 it is partly concealed at the insertion of the spiral valve, partly (myxi- 

 noids) in the liver. In these forms, owing to the complete disappearance 

 of the duct it becomes a gland of internal secretion. The pancreas 

 may be elongate, compact, or sometimes extremely lobulated. Usually 

 (fig. 241) it lies in a loop of the duodenum. From certain peculiarities 

 of structure the queston has arisen as to whether two distinct structures 

 are included in the pancreas. 



THE RESPIRATORY ORGANS. 



The respiratory organs have for their purpose the exchange of gases 

 between the blood and the surrounding medium water or air 

 carbonic dioxide being given off and oxygen being absorbed by the 

 circulating fluid. In order that the exchange be readily effected it is 

 necessary that the organs be richly vascular, that the walls between the 

 blood and the surrounding medium be extremely thin so as to permit 

 rapid osmosis, and that the osmotic surface be as great as possible. 

 Further, there must be an adequate mechanism for passing the oxygen- 

 containing medium over the respiratory surfaces. 



